From EP No. 121,911-A1, a filter membrane is known which consists of a polysulfone in the form of a hollow fiber, having a network structure across the entire thickness from the inner to the outer surface and in which the pores have a maximum pore diameter from 0.1 to 5 .mu.m and the pore orifices at the inner surface have a maximum diameter from 0.01 to 10 .mu.m and the orifices of the pores formed in the outer surface have a maximum diameter from 0.01 to 5 .mu.m.
Even though the pore structure is described as a homogeneous network structure or sponge structure, a broad distribution of the diameters of the orifices in the region of the outer wall surface is shown in an enlarged sectional illustration of the hollow fiber. The known membrane does not contain any skin in which only narrower pores than in the sponge structure exist, but contains widely different orifices, broken open outwards, of the network or sponge structure.
From EP No. 228,072-A2, a filter membrane is known wherein the polymer forming the membrane is as such hydrophobic and has a water absorption capacity of about 2 to 4%, and the membrane is hydrophillic, has a pore size from 0.02 .mu.m to 20 .mu.m and, at a given blow point, shows a high water flow velocity. Preferably, the polymer is polyether-sulfone and contains additions of polyethylene glycol or polyvinylpyrrolidone.
In the process for producing shaped articles having pores according to German Pat. No. 3,327,638, a porous polyamide-6 hollow fiber has been produced from a mixture of polyamide-6, .epsilon.-caprolactam and polyethylene glycol 300. Forming took place at a nozzle temperature of 210.degree. C. The spinning solution was homogeneous and of low viscosity and was therefore extruded into a U-shaped cooling tube, in which the mechanical loading, to which the polymer mixture is exposed up to the time of starting solidification, that is to say the start of dimensional stability, is kept small.
The membranes known from EP No. 121,911-A1 and EP No. 228,072A1 are formed from solutions of the polymer in aprotic solvents by known membrane formation processes. Examples of aprotic solvents are dimethylacetamide, dimethylformamide and N-methylpyrrolidone. The polymer contents of the solutions are decidedly low. For this reason, the viscosities are also low and thin-walled and mechanically less stable membranes are produced preferentially. The known membranes are virtually symmetrical, which has the consequence that the membrane flow decreases markedly with the thickness of the membrane.